Chromium alloys containing y{11 o{11 {11 and aluminium or silicon or both

ABSTRACT

Alloys for use as components of gas turbine engines have the following composition: Chromium at least 70% Yttrium and/or Y2O30.01% to 18% Aluminum up to 5% Silicon up to 8% Aluminium + silicon at least 0.01% The average spacing between yttrium and/or Y2O3-containing particles is preferably not more than 5 microns.

waited States Patent Jones et a1. Get. 15, 1974 1 CI'IROMIUM ALLOYSCONTAINING -2-" t.. F REIGN PATENT APPL T1 0 AND ALUMIWUIWOR SILICDN 1 29 G 8 ICA CNS 7 7 OR BOTH 06, 5 9/1 70 teat Britain 75/176 [75]Inventors: Rodney Charles Jones; Alan primary Examiner carl Quill-forth1 Abraham Hershman, both of Assistant Examiner-B. Hunt London EnglandAttorney, Agent, or FirmWenderoth, Lind & Ponack [73] Assignee: TheBritish Non-Ferrous Metals v I Research Association, London, 57]ABSTRACT England Alloys for use as components of gas turbine en ines g[22] Filed: Sept. 15, 1972 have the following composition:

[21] Appl' NO': 289,232 Chromium at least 70% Xlttrium and/or Y O 0.01%;2 18% 52 US. Cl 29/1s2.s, 75/176, 75/206 S11E11? 3513 8% [51] Int. ClB22f l/00 Muminium 091% [58] Field of Search 75/176, 206; 29/182, 182.5

The average spacing between yttrium and/or References Clted Y O-containing particles is preferably not more than UNITED STATES PATENTS5 microns.

3,159,908 12/1964 Anders, Jr. 75/206 3,466,155 9/1969 Watkins et al75/206 3 Clams No Drawmgs 3,703,369 11/1972 Seybolt 75/206 Thedevelopment of new materials for aircraft gas turbine applications isstimulated by the increase in efficiency obtained with higher operatingtemperatures. Advanced turbines are already operating with gas inlettemperatures close to the melting point of the nickelbased alloyscurrently used and this is only made possible by air cooling thecomponents. Engineering solutions of this type however incur increasedengine complexity and some sacrifice in power output, and a need existsfor a material capable of operating satisfactorily at highertemperatures. 4

The refractory metals (tungsten, molybdenum, tantalum and niobium) havevery high melting points and are extremely strong but all oxidise veryrapidly. Attempts to develop oxidation resistant coatings have so farproved unsuccessful.

Ceramics (e.g.. silicon nitride) have the disadvantage of being brittleat operating temperatures and probably necessitating a design thatmaintains the components in compression.

Chromium has only a moderately high melting point (1,850C), but hasreasonably good oxidation resistance, high elastic modulus, high termalconductivity and a comparatively low density. Chromium alloy developmenthas already produced simply alloys with higher creep rupture strengthsthan the highly alloyed nickeland cobalt-based alloys.

Chromium however has two main disadvantages:

1. It is generally brittle at ambient temperatures.

2. It readily absorbs nitrogen when heated in air at temperatures ofinterest in gas turbine applications (-l,lOC).

The embrittlement of chromium is associated, with the presence of tracesof nitrogen down to very low levels ppm). The problem can be eased byalloying with strong nitride formers (e.g., yttrium and other rare earthelements) and also by suitable warm working procedures. Solid solutionstrengthening with the refractory metals raises the ductile-brittletransition temperature, but dispersions of some oxides and carbides bothstrengthen chromium and improve the ductility.

The absorption of nitrogen at service temperatures is probably a moreserious problem. The nitrogen not only raises the ductile-brittletransition temperature, but also produces a hardbrittle chromium nitridelayer beneath the oxide scale. Additions of rare earth elements and ofmagnesium oxide have been shown to be effective in preventingnitridation under some conditions, but an alloy which is acceptable inservice has yet to be produced.

It is known that yttrium additions can reduce oxidation and prevent theformation of a nitride layer in slowly moving air at l,l00C. Protectionalong these lines is attributed to the incorporation of yttrium in thegrowing oxidescale, and requires concentrations of yt-' trium in excessof that which is soluble in chromium. However, the protection affordedby the yttrium is generally found to break down when the alloys aresubjected to the action of high-velocity air streams at l,l00C. Suchalloys are unsuitable forthe manufacture of gas turbine blades. lnchromium-yttrium alloys the yttrium is present as a chromium-yttriumeutectic which forms between primary chromium dendrites. In

a chill cast alloy the mean spacing between such eutectic regions is ofthe order of 30 microns.

The present invention provides a chromium-based alloy consisting of(apart from impurities):

chromium at least by weight yttrium up to 18% by weight YzOn up to 18%by weight aluminium up to 5% by weight silicon up to 8% by weightprovided that:

yttrium Y O at least 0.01% by weight aluminium silicon at least 0.01% byweight yttrium Y-zog aluminium silicon not more than 30% by weight.

These alloys are expected to have utility in gas turbine engines. Theinvention therefore includes components of gas turbine engines, andparticularly the moving or stationary blades of such engines, when madeof an alloy as defined above.

The alloys of the invention may be divided into three classes, namely:

a. alloys with added yttrium but no added Y O b. alloys with added Y Obut no added yttrium; and

c. alloys with added yttrium and added Y O Alloys in class a) have thefollowing composition:

chromium at least 70% by weight yttrium 0.01% to 18%, preferably 0.25%to 5.0% by weight aluminium up to 5% by weight silicon up to 8% byweight provided that:

aluminium +silicon at least 0.01%, preferably 0.25%

to 5.0% by weight yttrium aluminium silicon, not more than 30% byweight. The reasons why aluminium and silicon are capable of impartingimprovednitridation resistance to chromium-yttrium alloys. are notclear. They may, however, be

associated'withthe formation of intermetallic com-- pounds of the addedmetals with yttrium, which intermetallic compounds may melt at a higheror lower temperature than the chromium matrix. The following data may berelevant in this connection.

The alloys of this aspect of the invention may be prepared by aremelting the various ingredients together. Yttriumissubstantiallyinsoluble in chromium, and is normally present in yttrium-rich particlesdispersed in a chromiumlmatrix. The spacing of the yttrium particles isnot critical, but it is preferred that themean interparticle spacing iskept low, for example below 5 microns. If a very fine yttrium particlespacing is required, this may be achieved by plasma spraying the moltenmetal mixture in an inert or reducing atmosphere and splatorwater-quenching the molten droplets against a cooled metal surface.

Care has to be taken during a plasma spraying step to avoid excessiveoxidation or nitridation of the metal droplets. If the yttrium has notalready been oxidised during the splat casting and compacting steps, itmay be desirable to oxidize it deliberately in the resulting alloy, soas to prevent possible coarsening of the dispersed yttrium particles atelevated temperatures. Selective oxidation of the yttrium, leaving thechromium unaffected, may be effected by heating the alloy in acontrolled atmosphere, for example damp hydrogen.

When the alloy is formed by casting the molten metal, a subsequentextrusion step may result in an improvement of the properties, andparticularly of the nitridation resistance.

At least 0.01 percent by weight of yttrium is required to conferresistance to nitridation on the chromium Large quantities of yttriumaffect the properties of the chromium, and it is preferred to make theminimum addition of yttrium that is consistent with adequate protectionof the chromium. A preferred range is from 0.25 percent to 5.0 percentby weight of yttrium.

At least 0.01 percent by weight of aluminium and/or silicon is requiredto improve the resistance to nitridation of the alloy when subjected tohigh-velocity air streams at elevated temperature. More than 5 percentby weight of aluminium would make the alloy excessively brittle. Siliconproportions above 8 percent by weight confer less resistance tonitridation than do lower silicon concentrations.

EXAMPLE 1 The following alloys were prepared (percentages are by wight):

Cr 3% Y 1% Si Cr 3% Y 8% Si Cr 3% Y 2% A1 Cr 3% Y The alloys wereprepared by are melting and wer used as cast in all cases.

Specimens of these ternary alloys were exposed'for 48 hours at 1,100C.in slowly moving air; Weight gains were measured and all specimens wereexamined metallographically to determine the nature and degree ofoxidation and nitridation.

The results are summarised in Table 11. The oxide scale was very thin(-2 microns) but internal oxidation and internal nitridation (yttriumblackening) of the yttrium-bearing phase was observed in all cases andthe mean depth of penetration, measured from the specimen surface, isgiven.

The specific weight gains obtained are acceptable in all cases with theexception of unalloyed chromium.

TABLE 11 Specific Weight Gain and Magnitude of Attack on TernaryChromium Alloys after 48 hours in Slowly Moving Air at 1 100C Alloys inclass b) have the following compsition:

chromium at least by weight Y O 0.01% to 18% by weight. preferably 0.5%to 15% by volume aluminium up to 5% by weight silicon up to 8% by weightprovided that:

at least 0.01%, preferably 0.25% to 5.0% by weight Y O aluminium siliconnot more than 30% by weight.

aluminium silicon Chromium/Y O compositions, though not strictly alloys,are called alloys in this specification. The justification for thisterminology is that it does not appear to matter whether the yttrium ispresent as the metal or the oxide as protection against nitriding isafforded in each case, and that, as yttrium metal is readily oxidised,some Y O is almost inevitably present, or forms during exposure to airat high temperatures.

The Y O is intimately dispersed in the chromium matrix, as discreteparticles preferably with a mean interparticle spacing of not more than5 microns and optionally not more than 3 microns. Sub-microninterparticle spacings are desirable but are difficult to achieve usingpowder metallurgy techniques.

It will be appreciated that the size and spacing of the Y O particles inthe chromium matrix will be related to the total Y O content. Thus, fora given Y O particle size there will be a minimum total Y O contentrequired to achieve a mean interparticle spacing of, say, 3 microns,which minimum may well be substantially greater than the 0.01 percentspecified above.

These alloys may be prepared with the desired interparticle spacing bycompacting mixtures of chromium powder, aluminium and/or silicon powder,and yttria powder. Chromium is available commercially as a powder ofnominally 5 micron size, and we have elutriated thisto obtain powdersexclusively below 4 microns, mostly below 3 microns, and having anaverage size of about 2 microns.

Yttrium oxide is commercially available as a powder of 2 to 3 micronsize. We have elutriated this to obtain a powder of approximately 1micron size. Fine Y O powders having a particle size of around 0.1micron can be prepared by the controlled calcination of yttrium oxalate.

Alternatively, the aluminium and/or silicon may be incorporated in thechromium powder by are melting the chromium aluminium and/ or siliconmixture and grinding the resulting alloy to a fine powder.

These powders may be mixed and compacted by known techniques. Forexample, an unpressed charge of the mixed powders may be treated inhydrogen at 1,250C to reduce the concentration of Cr O and nitrogenimpurities; pressed at ambient temperature into a billet; sintered inhydrogen at up to 1,400C; machined, canned, and extruded to give a barof 97 percent theoretical density.

When the initial charge consisted of the powders as purchased andcontained percent by volume of Y O the alloy contained a uniformdispersion of Y O particles in the transverse direction at a meaninterparticle spacing of 3.6 microns.

When the initial charge consisted of the finer elutriated powders andcontained 10 percent by volume of Y O the mean transverse spacingbetween the Y O particles was 2.6 microns.

The proportion of yttrium oxide in these alloys is preferably from 0.02percent to 25 percent by volume, particularly from 2 percent to percentby volume, on the total volume of the alloy.

ixAMPLE i A C r-067o Al-l0 alloy billet was protiice d from the fines (3microns) obtained by elutriating asreceived chromium powder, fine (1micron) elutriated Y O and 1 percent aluminium in the form of 3 micronCr-l0%Al powder. A 1 percent aluminium addition was made to allow forsintering.

The Cr-10%A1 powder was produced by arc-melting chromium flake with pureAl foil, crushing, ball milling, sieving and elutriating the sub-37micron fraction.

The mixed powders were elutriated at an air velocity sufficient toentrain all the individual powder particles but not large agglomerates.This, combined with the vigorous agitation in the fluidised bed, shouldensure adequate mixing and a fine oxide dispersion.

After pre-reduction of the loose powder mixture for 48h at 1,200C inflowing hydrogen an extrusion billet was produced by isostaticcompaction at 550 N/mm for 10 minutes, sintering in hydrogen for 24h at1,200C followed by 48h at 1,400C and finally extruding in an evacuatedmild steel can at 1,200C with an extrusion ratio of 13:1 (16 mm dia.product). The extrusion was completely sound.

This alloy contains a fine, uniform oxide dispersion with a meaninterparticle spacing of 2.8 micron transverse to the extrusiondirection.

On exposure to slowly moving air for48h at 1,100C a thin, adherent oxidescale formed, which did not spall on cooling. A Cr N layer was notdetected by either metallographic examination or X-ray diffractionanalysis. The weight gain measured was 0.73 mg/cm (cf.

possible aluminium losses during On exposure to high-velocity air for48h at 1,100C a thin adherent oxide scale was produced with a powderycovering on the impinged face. Again spalling did not occur on coolingand a Cr N layer could not be detected.

X-ray powder diffraction analysis showed that the powdery oxidecontained Cr O and yttrium aluminate probably of the type Y AI OGlancing incidence analysis of the impinged surface after removal of thepowderyoxide revealed the presence of Cr, Cr O and extra lines at d=3.8,3.34 and 1.87A.

i xAMPLEE A vol% Y203 scribed in Example 2, using a Cr-5% Si masteralloy powder.

Alloys in class e) have the following composition:

chromium at least 70% by weight Yttrium 0.01% to 18%, preferably 025% to5.0%, by weight Y O 0.01% to 18% by weight. preferably 0.5% to 15% byvolume aluminium up to 5% by weight silicon up to 8% by weight providedthat:

not more than 29.9%, preferably not more than 18%, by weight m at least0.1% by weight, preferably 0.25% to 5.0% by weight yttrium Y O aluminiumsilicon not more than 30% by weight.

aluminium silicon was extruded at 1,200C with an extrusion ratio of 13:1

(16 mm dia. product). The extrusion exhibited some hot-tearing at theback end but the majority of the product was sound.

This alloy contained a coarse oxide dispersion with a meaninterparticlespacing of 9.0 microns transverse to the extrusion direction.

As-extruded specimens exposed to slowly moving air for 48h at 1,100Cexhibited a thin adherent oxide scale and a weight gain of 1.8 mg/cm NoCr N layer was identified metallographically.

The behaviour of the alloy in high velocity air was similar to that ofthe Cr-0.6%Al-l0 vol% Y O alloy (Example 2). Again, a thin, adherentoxide scale was formed with a covering of a powdery oxide and there wasno evidence of the formation of a Cr N layer.

Specimens of certain alloys according to this invention were rotated ina squirrel cage in the high temperwas produced as deature, highvelocity, exhaust gasses from a gas turbine combustion unit. The air wassupplied by a centrifugal blower at a mass air flow of 60 lb/min. Thefuel used was standard aviation kerosene burnt at an air:fuel ratio of30:2. The temperature of the hottest part of the specimens (centre) wasmaintained at l,lC and continuously monitored by a radiation pyrometerpreviously calibrated against an optical pyrometer.

The complete test consisted of 48h at l,l00C including l7 thermal cyclesto room temperature. Heating and cooling during cycling was very rapid,being completed in approximately 1 minute.

The specimens were then examined, particularly to determine the depth ofpenetration of the chromium nitride layer and of the internalnitridation of the yttrium-rich phase. The results of two sets of testsare set out in Table III below.

1 penetration of Cr N less than 5 microns, or penetration of YN lessthan 200 microns 2 penetration of Cr N greater than 5 microns, or

penetration of YN greater than 200 microns. We claim: 1. Achromium-based alloy consisting of (apart from impurities):

chromium at least by weight yttrium up to l8% by weight Y O up to l8% byweight aluminium up to 5% by weight silicon up to 8% by weight providedthat:

yttrium Y,O at least 0.01% by weight aluminium silicon at least 0.01% byweight yttrium Y O aluminum silicon not more than 30% by weight.

2. A chromium based alloy consisting of, apart from impurities; thefollowing components chromium: at least 70% by weight, Y O 0.01% to 18%by weight, aluminium: up to 5% by weight. silicon: up to 8% by weight,

provided that:

aluminium silicon is at least 0.01 percent by weight,

Y O aluminium silicon is not more than 30 percent by weight, and

wherein the Y O is in the form of particles having an average size ofless than 3 microns and an average inter-particle spacing of less than 5microns.

3. An alloy as claimed in claim 2 containing from 0.5

percent to 15 percent by volume of Y O UNITED STATES PATENT OFFICECERTIFICATE OF CORRECTION Patent N Dated October lnventofls) RodneyCharles Jones ,Alan Abraham Hershman It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

IN THE CLAIMS:

Column 8 cancel claim 1 without prejudice to the subject matter thereof.

Renumber claim "2" as l Renumber claim "3" as 2 Claim 3, (now renumberedas claim 2) first line,

change "2" to l Signed and Scaled this fourteenth Day Of October 1975[SEAL] Arrest:

RUTH C. MASON C. MARSHALL DANN Arresting Officer (ummisxiuner of Patentsand Trademarks F ORM P0-105O (10-69) USCOMM-DC 60376-P59 k U.S.GOVERNMENT PRINTING OFFICE: I969 O-365-334,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent 3,841,847 Dd October 15,1974

' lnventofls) Rodney Charles Jones ,Alan Abraham Hershman It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

IN THE CLAIMS Column 8, cancel claim 1 without prejudice to the subjectmatter thereof.

Renumber claim "2" as l Renumber claim 3" as 2 Claim 3, (now renumberedas claim 2) first line,

change "2" to l Signed and Bealcd this fourteenth Day of October 1975[SEAL] Arrest:

. RUTH C. MASON C. MARSHALL DANN Arresting Officer (mnmissiuner ofPatents and Trademarks FORM PO-1050 (10-69) USCOMM-DC 60376-P69 1% U5,GOVERNMENT PRINTING OFFICE: i969 0-366-334,

1. A CHROMIUM-BASED ALLOY CONSISTING OF (APART FROM IMPURITIES):
 2. Achromium based alloy consisting of, apart from impurities; the followingcomponents
 3. An alloy as claimed in claim 2 containing from 0.5 percentto 15 percent by volume of Y2O3.